The Map Reading Handbook
A contour is a level line: join every point at one height, then repeat at a fixed step, and the flat sheet begins to carry hills.
The flooding-island thought experiment makes the idea concrete. Let the water rise by exactly the contour interval and mark each new shoreline from above; those nested lines are the contours. The interval — ten metres on many walking sheets — is stated in the margin and never changes within a sheet.
Spacing is steepness. Where contours crowd, a short horizontal step gains a full interval of height, so the slope is severe; where they wander far apart, the ground is gentle. Every fifth line is usually a heavier index contour carrying a height figure, so the reader can count without losing place.
Forms repeat as a small vocabulary. Closed loops are hills or knolls; if the loop carries hachure ticks pointing inward, it is a depression instead. Where contours bend into a V at a stream, the point of the V aims uphill — the rule of V — while the same bend on a spur aims down.
On narrow screens, swipe or scroll the plate sideways.
Between two summits the contours pinch into an hourglass: that waist is a saddle, the classic crossing point of a ridge. Where several contours merge into one inked line, the ground is vertical — a cliff. Contours otherwise never cross, because one point cannot sit at two heights unless the rock overhangs.
To turn lines into a mental picture, build a quick profile: lay a paper edge along the route, tick where each contour crosses with its height, then plot those ticks and join them. The silhouette that results is the walk as the legs will feel it.
Relief is also read with the map's other clues: spot heights and triangulation pillars fix exact points, streams always run in valleys, and the blue drainage network sketches the skeleton that the contours flesh out.
Further reading